Commit c0cb87fb unwisely introduced a dependency on the StringInfo
machinery in fe-connect.c.  We must not use that in libpq, because
it will do a summary exit(1) if it hits OOM, and that is not
appropriate behavior for a general-purpose library.  The goal of
allowing arbitrary line lengths in service files doesn't seem like
it's worth a lot of effort, so revert back to the previous method
of using a stack-allocated buffer and failing on buffer overflow.

This isn't an exact revert though.  I kept that patch's refactoring
to have a single exit path, as that seems cleaner than having each
error path know what to do to clean up.  Also, I made the fixed-size
buffer 1024 bytes not 256, just to push off the need for an expandable
buffer some more.

There is more to do here; in particular the lack of any mechanical
check for this type of mistake now seems pretty hazardous.  But this
fix gets us back to the level of robustness we had in v13, anyway.

Discussion: https://postgr.es/m/daeb22ec6ca8ef61e94d766a9b35fb03cabed38e.camel@vmware.com